The following account of the prior art relates to one of the areas of application of the present application, hearing instruments.
A hearing instrument providing the best sound quality experience will usually have a microphone at the entrance to the ear canal. This microphone location ensures that the naturally occurring information in the sound can be preserved—i.e. frequency spectrum, directional cues, interaural time difference, etc. At the same time, the implementation of a directional microphone system necessitates using microphones spaced some distance apart, typically 1 cm. A directional system offers the advantages of adaptive directionality contributing to the separation of wanted from unwanted sounds as long as they are emitted from different locations in space and in different directions (as seen from the hearing instrument). However, a directional system is known to disturb the basic sound quality of the signal-frequency colorations, phase changes and an increase in system noise are introduced by the directional system.
Using a microphone in or at the entrance to the ear canal is accepted in small hearing instruments such as completely in the canal (CIC) hearing instruments. It is well known, that such small hearing instruments have a limited amplification before acoustic feedback occurs and that the ventilation channel must also be of limited diameter such as less than perhaps 2 millimeters, depending on the hearing loss and on the use of a digital feedback cancellation system. A vent of only limited diameter may, however, cause occlusion problems for a substantial number of users.
Hearing instruments with one or more microphones located behind the pinna are often used in combination with a larger vent due to the limited risk of feedback with this more distant microphone location (distant from the speaker outlet). This microphone position behind the pinna does, however, limit the natural sound quality since the influence of the pinna on the sound is largely ignored.
US2010150385A1 deals with feedback prevention by arranging a directional microphone comprising two electrically interconnected microphones, and a receiver along a straight line. The directional effect of the directional microphone is set such that, when viewed from the directional microphone, the receiver is arranged in the direction of the lowest sensitivity of the directional microphone.
US2008107292A1 deals with an optically unnoticeable and acoustically improved behind-the-ear hearing device having a housing which can be worn behind the ear, a signal processing facility which is arranged in the housing, and which comprises an optoelectrical converter. Further, at least one optical microphone—arranged outside the housing—and connected to the signal processing facility by way of an optical wave guide is provided. The optical microphone can be positioned in the concha or in the auditory canal.
U.S. Pat. No. 7,471,799B2 deals with a method of noise reduction in a hearing aid or a listening device to be used by a hearing impaired person in which the noise reduction is provided primarily in the frequency range wherein the hearing impaired has the smallest hearing loss or the best hearing. Preferably, the noise reduction is achieved through beamforming of the signals from some or all of the microphones and whereby the number of microphones and their spacing is such that the highest directivity is provided in the frequency range, wherein the hearing impaired has the smallest hearing loss.